纺锤体组装检验点

纺锤体组装检验点 Table 1Components of spindle assembly checkpoint Core SAC proteinsFunction Mad1 (mitotic-arrest deficient-1)Phosphorylated Mad1 facilitates recruitment of Mad2 to kinetochores unattached tomicrotubules or lack of tension between sister chromatids. Forms a complex with Cdc20, Mad2 and Mad3/BubR1 to inhibit APC/C activity To form the MCC with Bub3, Cdc20 and BubR1/Mad3 to inhibit the APC/C activity. Mad2 (mitotic-arrest deficient-2)Monitors attachment of microtubules to kinetochores One component of the MCC, monitoring the tension between sister chromatids. Mad3/BubR1 (mitotic-arrest deficient-3)A protein kinase that phosphorylates Cdc20 and recruits other checkpoint proteins Bub1 (budding uninhibited by benzimidazole-1) such as Mad2, BubR1 and CENP-E to kinetochores A protein kinase that phosphorylates Mad1, which is required for recruitment of other SAC proteins to kinetochores Mps1 (multipolar spindle-1)Form a complex with Mad1, Mad2, Cdc20 and BubR1/Mad3 to inhibit the APC/C. E3 ubiquitin ligase that targets mitotic regulators such as securin, cyclin B, shugoshin Bub3 (budding uninhibited by benzimidazole-1)for proteolysis by 20S proteasomes when activated by Cdc20 APC/C (anaphase promoting complex/cyclosome) A kinesin family member; Binds to BubR1 to stimulate its kinase activity and is required for capture and stabilization of microtubules at kinetochores in prometaphase. CENP-E (centromeric protein E)An APC activator Cdc20 (cell division cycle-20 ) [12] Fig.1Spindle assembly checkpoint on and off (modified from reference ) 成 C-Mad2-Cdc20, 从而抑制 APC/C 的活性; 胞质中从而保证分裂后期染色体能正确分配到子代细胞中。的 O-Mad2也可与C-Mad2-Cdc20 结合, 并转变为C- 当着丝点和纺锤体微管正确连接并在姐妹染色Mad2。通过这种级联放大作用, 在着丝点上形成大量 单体间产生合适张力后, 着丝点上的 Mad1-C-Mad2 的 Mad2-Cdc20, 从而有效抑制 APC/C 的活性, 阻止 二聚体, 被动力蛋白(dynein)沿微管运向纺锤体的两 [15,16]细胞周期从中期进入后期。而胞质中的 Mad1-C- 。Mad1-C-Mad2 移离着丝点后, 即导致 Cdc20 极comet Mad2因为结合了P31蛋白而不能再募集O-Mad2, 从 C-Mad2-Cdc20 复合体上释放出来。游离的 Cdc20 [2]因此只有在未连接微管的着丝点上可以产生这种级 与 APC/C 结合使后者活化。活化的 APC/C 通过泛 [2,14] 联放大作用。素化介导的蛋白质降解途径使securin降解, 从而释 Bub1 和 BubR1 主要感受姐妹染色单体上着丝点 放出separase。游离的separase可降解连接姐妹染色 [13]间的张力, 促使与纺锤体微管错误连接的姐妹染色 单体着丝粒的cohesin蛋白1 (sister chromatid cohe- 单体上的着丝点分别与来自纺锤体两极的微管连接, sion protein 1, SCC1), 从而使姐妹染色单体分开; 同 4减数分裂SAC异常 近年大量的研究表明, 减数分裂 SAC 活性异常[51]可能导致染色体错误分离、细胞周期紊乱等。在 Mad2 RNAi的小鼠初级卵母细胞中, 细胞周期蛋白B 和securin发生提前降解, 并导致非整倍体发生率升 [52] 高; 在mad2小鼠的卵母细胞中, 次级卵母细胞的 非整倍体发生率也有所升高, 并会引起雌鼠生育力下 [53]降。在BubR1表达不足小鼠中, 生殖细胞非整倍体 [54] 发生率明显升高, 且小鼠不育。与Mad2、BubR1表 达下调类似, 向小鼠卵母细胞注射Bub1抗体也会导 Fig.2 Attachmentofsisterchromatidkinetochorestospindle[55]microtubles at prometaphase in meiosis I, with sister 致分裂后期提前发生。此外, SAC蛋白过表达也会 chromatid kinetochores of each chromosome pair co- [53]导致减数分裂染色体分离异常, Niault等发现, Mad2 connecting to one spindle pole 过表达的小鼠卵母细胞同样会发生减数分裂I染色体 错误分离, 而产生非整倍体次级卵母细胞。上述结果 表明, Mad2、BubR1 和 Bub1 等在生殖细胞中也是保 证染色体正确分离的关键蛋白, 其含量的改变会导致 [56]SAC 功能异常。 众所周知, 在女性中, 非整倍体配子发生率随年 [3,57,58]龄增长会明显升高。尽管非整倍体的发生率随 年龄增长而升高可能有多种原因, 如姐妹染色单体间 [59,60]cohesins的提前解离, 但新近的研究发现, 这也可 [61] 能与SAC蛋白表达量的下降有关。如Steuerwald等 发现在年老女性的卵母细胞中, Mad2 的 mRNA 含量 [62][63]降低; Baker等和Hamatani等分别在小鼠卵母细 胞中发现, BubR1 和 Bub1 的表达也会随年龄增加而 有所下降。而这些蛋白质表达下降就可能引起 SAC Fig.3 Attachmentofsisterchromatidkinetochorestospindle[56]活性降低。因此, 在年老的女性中, SAC功能减弱 microtubles at prometaphase in meiosis II, with sister 就无法保证染色体的正确分离, 而导致非整倍体的发 chromatid kinetochores of each chromosome pair co- 生率升高。 connecting to different spindle poles shugoshin和PP2A发生移位, 使得cohesins能被磷酸[46,47] 化并被separase降解(图3)。Bub1对shugoshin与 着丝粒的结合起着重要作用, 在酵母中, Bub1的N端 与shugoshin1连接, 而C端的激酶结构域与shugoshin2 5 展 望 共同作用促进减数分裂I姐妹染色单体上的着丝点与 自发现以来, 纺锤体组装检验点一直是细胞生物 [48]来自纺锤体同一极的微管连接。 学和遗传学研究的热点, 但其分子机制至今未完全明 总之, 在哺乳动物卵母细胞中, SAC无论在减数 了。如细胞质中的 O-Mad2 是如何与 C-Mad2 作用并 分裂I还是减数分裂II, 对于保证染色体的正确分离 促使前者发生构象改变并与 Cdc20 结合目前仍不清 [2]都是必需的。但需要指出的是, 尽管有时SAC蛋白表 楚。SAC异常与非整倍体、肿瘤和出生缺陷的发生 达及功能正常, 却仍然无法有效地防止染色体的异常 密切相关, 了解SAC的机制对于有效防止先天出生缺 分离, 如在联会复合体组分scp3敲除的小鼠中, 非整 陷, 以及非整倍体肿瘤的预防、诊断和治疗具有重要 倍体的次级卵母细胞发生率明显增加; 这可能由于 作用。 scp3形成的单价体上的姐妹染色单体着丝点分别与 纺锤体两极的微管连接, 从而蒙骗了SAC的监控, 而 导致姐妹染色单体在减数分裂I发生了提前分离并最 参考文献(References)[49,50]终形成非整倍体卵子。 Kops GJ, Weaver BA, Cleveland DW. 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MAD2 prevents Hamatani T, Falco G, Carter MG, et al. Age-associated alter- aneuploidy and premature proteolysis of cyclin B, securin dur- ation of gene expression patterns in mouse oocytes, Hum Mol ing meiosis I in mouse oocytes, Genes Dev, 2005, 19(2): 202- Genet, 2004, 13 (19): 2263-2278 Sp ind le Ass emb ly Ch eck poi nt Zheng Wang, Xiao-Hua Jiang, Huan Zhang, Qing-Hua Shi* (Hefei National Laboratory for Physical Sciences at Microscale, School of Life Sciences, University of Science and Technology of China, Hefei 230027, China) AbstractThe spindle assembly checkpoint monitors attachment of microtubles to chromosomes, and the proper tension between sister chromatids during mitosis or homologous chromosomes duringmeiosis, to ensure equal distribution of chromosomes into daughter cells. Dysfunction in the spindle assembly checkpoint could lead to the generation of aneuploidy, which is closely related to diseases such as cancer, spontaneous abortions or birth with defects. Key wordsspindle assembly checkpoint; aneuploidy; mitosis; meiosis; cancer This work was su pported by th e Program of “One H undred Talented Peo ple” (N o.KJ20 7004) and the Proje ct of K nowled ge Inn ova- tion (No .K SCX1 -Y W-R-51 ) of Chines e Ac ad em y of Scien ce s, the Na tion al Bas ic Res ea rc h Prog ra m o f Ch in a (N o.20 07 CB94 74 01 ), the Na tio na l H igh Te chn ology Re sea rc h a nd Dev elopm ent Program of Ch ina (863 Pro gram) (No.20 06A A0 2Z4 B4 ), an d the Na tiona l Natural Sc ie nc e Fo un da tion (No .3 05 71 030 , No .3 06 71 16 8 an d No .3 07 25 01 3) *Co rres pond ing auth or. Tel/Fax: 86-551-3600 344, E-ma il: qsh i@us tc.edu.cn